The human body has an amazing ability to heal itself when hurt in certain ways. For example, when your body incurs a minor cut or scrape, various complex mechanisms activate that heal and repair the damage. Generally speaking, however, our machines are unable to do the same.
It is highly desirable to reduce maintenance costs by minimizing explicit preemptory maintenance and to prevent catastrophic failures. An ultimate goal is to monitor the integrity of the structure in operating conditions during its entire working life. The development of in-service structural health monitoring (SHM) and damage detection techniques has attracted a large number of academic and industrial researchers.
Once damage is detected during operation of the structural platform, in general, a management process is performed through damage identification to determine whether to continue operation or to stop operation in order to perform structural repair.
Different kinds of damage may occur due to severe operational conditions. For example, damage can be caused by fatigue, erosion, corrosion, impact, moisture and/or other effects. The operational life cycle of a structural platform can be significantly reduced. In some cases, the entire structural component must be replaced instead of being repaired.
Seeking safety improvement, reduction of maintenance cost and human error, efforts are underway to develop automatic SHM systems capable of inspecting and detecting damages in real time without need for human interference or attention. Therefore, new SHM technologies will lead to early detection of damage that often in the past was identified only through scheduled manual inspections.
In general, “self-healing material” defines those materials that in the presence of damage can self-repair spontaneously or with the aid of a stimulus, and thus maintain its functionality or otherwise continue to function. The literature shows that different strategies and approaches have been investigated to provide this feature in all classes of materials including for example polymers, metals and ceramics.
The concept of self-healing of damage in materials is of great interest to the industry particularly in the following applications in structural platforms:                Structural components for which reliability, even in overload conditions, is of critical importance;        Surfaces where damages are not allowed, as in anti-corrosive coatings, decorative paints and thermal barrier coatings;        Structural components inaccessible or access difficult for inspections and repairs;        Structural components that require long life;        Other.        
One useful strategy in self-healing polymers has been the incorporation of microcapsules or hollow glass fibers that, when broken, release a healing agent. Other mechanisms, such as the use of micro-vascular networks, have also been used. In the case of polymer matrix materials that have intrinsic self-healing, an external stimulus is required, e.g., heating is applied.
For self-healing metal, a primary focus in the past has been on the technologies of coatings applied to surfaces of metal alloys. Research conducted in scientific and technological bases on the subject of self-healing metal showed a low number of examples of application of this concept as compared to some other classes of materials.
For ceramic materials, although the typical self-healing process requires high temperature, the regeneration of properties at lower temperatures can be obtained if the grain boundary contains a vitreous phase. Such ceramic systems are able to surpass inherent problems of traditional ceramics, i.e., low fracture toughness, sensitivity to thermal shock, mechanical stiffness and low reliability.